package easy.trace.core.util;

/**
 * @author easy
 */
public class IdGen {
    private long workerId;
    private long dataCenterId;
    private long sequence = 0L;
    /**
     * Thu, 04 Nov 2010 01:42:54 GMT
     */
    private long twepoch = 1288834974657L;
    /**
     * 节点ID长度
     */
    private long workerIdBits = 5L;
    /**
     * 数据中心ID长度
     */
    private long dataCenterIdBits = 6L;
    /**
     * 最大支持机器节点数0~31，一共32个
     */
    private long maxWorkerId = -1L ^ (-1L << workerIdBits);
    /**
     * 最大支持数据中心节点数0~31，一共32个
     */
    private long maxDataCenterId = -1L ^ (-1L << dataCenterIdBits);
    /**
     * 序列号12位
     */
    private long sequenceBits = 12L;
    /**
     * 机器节点左移12位
     */
    private long workerIdShift = sequenceBits;
    /**
     * 数据中心节点左移17位
     */
    private long dataCenterIdShift = sequenceBits + workerIdBits;
    /**
     * 时间毫秒数左移22位
     */
    private long timestampLeftShift = sequenceBits + workerIdBits + dataCenterIdBits;
    /**
     * 4095
     */
    private long sequenceMask = -1L ^ (-1L << sequenceBits);
    private long lastTimestamp = -1L;

    private static final long DEFAULT_WORKER_ID = BuriedPointMachineUtil.getLastIP();
    private static final long DEFAULT_DATA_CENTER_ID = Long.valueOf(BuriedPointMachineUtil.getProcessNo());


    private static class IdGenHolder {
        private static final IdGen instance = new IdGen();
    }

    public static IdGen get() {
        return IdGenHolder.instance;
    }

    public IdGen() {
        this(DEFAULT_WORKER_ID, DEFAULT_DATA_CENTER_ID);
    }

    public IdGen(long workerId, long dataCenterId) {
        this.workerId = Math.abs(workerId % (maxWorkerId + 1));
        this.dataCenterId = Math.abs(dataCenterId % (maxDataCenterId + 1));
    }

    public synchronized long nextId() {
        long timestamp = timeGen();
        //获取当前毫秒数
        //如果服务器时间有问题(时钟后退) 报错。
        if (timestamp < lastTimestamp) {
            throw new RuntimeException(String.format("Clock moved backwards.  Refusing to generate id for %d milliseconds", lastTimestamp - timestamp));
        }
        //如果上次生成时间和当前时间相同,在同一毫秒内
        if (lastTimestamp == timestamp) {
            //sequence自增，因为sequence只有12bit，所以和sequenceMask相与一下，去掉高位
            sequence = (sequence + 1) & sequenceMask;
            //判断是否溢出,也就是每毫秒内超过4095，当为4096时，与sequenceMask相与，sequence就等于0
            if (sequence == 0) {
                timestamp = tilNextMillis(lastTimestamp);
                //自旋等待到下一毫秒
            }
        } else {
            sequence = 0L;
            //如果和上次生成时间不同,重置sequence，就是下一毫秒开始，sequence计数重新从0开始累加
        }
        lastTimestamp = timestamp;
        // 最后按照规则拼出ID。
        // 000000000000000000000000000000000000000000  00000            00000       000000000000
        // time +  dataCenterId + workerId  +  sequence
        // 所有 | 等价于 +
        return ((timestamp - twepoch) << timestampLeftShift) |
                (dataCenterId << dataCenterIdShift) |
                (workerId << workerIdShift) |
                sequence;
    }

    protected long tilNextMillis(long lastTimestamp) {
        long timestamp = timeGen();
        while (timestamp <= lastTimestamp) {
            timestamp = timeGen();
        }
        return timestamp;
    }

    protected long timeGen() {
        return System.currentTimeMillis();
    }

}
